In conventional techniques, an adhesive composition including an acryl-based polymer is used to bond various optical films such as anti-reflection films and electrically-conductive films, to display panels such as liquid crystal display devices.
The refractive indices of materials commonly used to form optical films or optical components are typically as follows: about 1.52 for glass, about 1.51 for methacrylic resin, and about 1.60 for polycarbonate.
Unfortunately, after dried or cured, such a conventional adhesive composition has a refractive index of about 1.47. This causes a difference in refractive index at the interface between the adhesive layer and an optical film or an optical component, and due to the difference, total reflection of shallow-angle light can occur and reduce the light extraction efficiency.
Another conventional adhesive composition contains, as a main component, a product of copolymerization of an acryl-based polymer with an aromatic ring-containing copolymerizable monomer. Such a conventional adhesive composition has a refractive index higher than that of the above conventional acryl-based adhesive composition, but does not have well-balanced other properties.
Some efforts have been made to improve the physical properties of pressure-sensitive adhesives or polymer compositions, such as mechanical strength, by adding various fine particles to pressure-sensitive adhesives or polymer compositions used for a wide variety of applications. Methods commonly used include uniformly dispersing surface-treated fine particles in a composition or uniformly dispersing fine particles in a composition by a process using any of various dispersing machines.
For example, any of various silane coupling agents is added to a colloidal silica gel of 30 nm or less to modify the surface of the silica, and the modified silica gel is mixed with a polymer solution to form a pressure-sensitive adhesive, in which the fine particles reduce the volumetric shrinkage rate and thus should be effective in increasing the adhesive force (see Patent Document 1).
It is disclosed that a diffusion pressure-sensitive adhesive containing nanoparticles is provided to improve contrast or viewing angle (see Patent Document 2). Unfortunately, the document does not show any specific method for dispersion and stabilization.
A method of preparing an adhesive syrup is also disclosed, which includes subjecting metal oxide particles of 200 nm or less to a surface treatment with a long-chain aliphatic acid or an organosilane as a surface modifier and dispersing the modified particles in a monomer (see Patent Document 3).
An ultraviolet-blocking pressure-sensitive adhesive is also disclosed, which is a coating composition including a mixture of zinc oxide fine particles of 0.2 μm or less and an acryl-based polymer produced by copolymerization using a hydroxyl group-containing monomer in an organic solvent that does not produce any acid functional group (see Patent Document 4).
Unfortunately, when fine particles prepared using the surface treatment method are dispersed in various compositions, the fine particles can aggregate into a complex depending on the type of the polymer, which can produce cloudiness and a certain level of haze. Thus, the type of the solvent, the surface modifier, or the coupling agent must be changed as needed depending on the composition of the polymer. In fact, laborious screening is necessary, in which various examinations are performed for optimization. In some cases, there are associated problems such as impossibility to use general-purpose solvents.
There is also disclosed an adhesive composition containing a specific content of metal-based fine particles and an aromatic polymer including an aromatic ring-containing monomer component (see Patent Document 5).
Patent Document 1: JP-A-2005-255706
Patent Document 2: JP-A-2005-301213
Patent Document 3: JP-A-2003-513122
Patent Document 4: JP-A-2005-213482
Patent Document 5: JP-A-2010-43156